(1) Field of the Invention
The present invention relates to a liquid crystal display device, and in particular, to an IPS type liquid crystal display device where an electrical field is applied parallel to the surfaces of the substrates.
(2) Description of the Related Art
Liquid crystal display devices which are referred to as an IPS (in-plane switching) type are liquid crystal display devices where liquid crystal molecules are aligned horizontally to the surface of the panel and are rotated by 90 degrees within the horizontal surface when an electrical field (lateral electrical field) parallel to the panel surface is applied. In this IPS type liquid crystal display device, common electrodes are formed on the first substrate side where video signal lines (drain lines), scan signal lines (gate lines), thin film transistors and pixel electrodes are formed, and the liquid crystal layer is driven by the electrical field in a direction within the plane of the first substrate that is caused by the difference in the electrical voltages applied to the pixel electrodes and the common electrodes. In the IPS type liquid crystal display device having such a structure, pixel electrodes in linear form are arranged in a layer above the common electrodes in plane form that are formed of a transparent conductive film with an insulating film in between so as to overlap the common electrodes. This structure allows an electrical field to be created in the direction of the normal to the first substrate in a layer above the electrodes in linear form or in portions between adjacent electrodes in linear form, and therefore, liquid crystal molecules incline instead of being parallel to the panel surface, and thus, it is known that the display mode efficiency lowers.
In recent years, the performance of liquid crystal display devices has been increased, and such products that make a WVGA display of 800×480 pixels possible have been demanded as middle to small-sized liquid crystal display devices of three to four inches. In middle to small-sized liquid crystal display panels where a WVGA display is possible, however, it is necessary to form a number of display pixels (hereinafter referred to as pixels) within a limited display region, and therefore, the width of one pixel becomes approximately 30 μm. As a result, a further increase in the aperture ratio and in the display mode efficiency has been demanded.
An example of such a liquid crystal display device where the display mode efficiency has been increased is described in JP 6-214244A. In this liquid crystal display device in JP 6-214244A, a pair of electrodes are formed at the two ends of a pixel region, and a video signal is supplied to one electrode (pixel electrode, source electrode) and a common signal is supplied to the other electrode (common electrode) as a reference so that an electrical field that is parallel to the main surface of the liquid crystal display panel (a so-called lateral electrical field) is created to drive the liquid crystal molecules in the structure. In the liquid crystal display device in JP 6-214244A in particular, the pixel electrodes and the common electrodes are formed in a wall form so as to protrude from the main surface of the first substrate towards the second substrate, and at the same time, the direction in which they run is perpendicular to the main surface of the first substrate. As a result of this structure, the liquid crystal display device in JP 6-214244A makes the density of the lines of electric force uniform in regions close to and far away from the first substrate (in regions far away from and close to the second substrate), and thus, the structure allows the display mode efficiency to increase.